Determination of Seismic Velocities in Bentonite Backfill Through Ultrasonic Measurements in the FE-Experiment

One important aspect of implementation of deep underground radioactive waste repositories is the physical characterization of backfill material which fills the voids between waste containers and ambient host rock. The Mont Terri Rock Laboratory, located in NW Switzerland and operated by swisstopo, is an international subterranean research facility which offers a perfect environment to carry out experimental test phases on such barrier systems. In this study, we report on eight years of ultrasonic monitoring in the bentonite backfill of the FE-Experiment, which aims at monitoring long-term changes in material properties of the backfill around a simulated radioactive waste repository under the conditions of rising temperature and increasing humidity. The FE Experiment is a full-scale heater test in the Opalinus clay. It simulates the construction, waste emplacement, backfilling and early post-closure evolution of spent fuel (SF)/vitrified high level waste (HLW) repository tunnel (Mueller et al., 2017).

 

The monitoring method is based on the generation of seismic signals in the frequency range of 1000 Hz to 100 kHz emitted and received by acoustic sensors. In total, 17 sensors (4 emitters and 13 receivers) manufactured by GmuG, form two different arrays. One is a permanent array installed within the bentonite material itself, responsible for measuring the ultrasonic transmission on a cross section through the tunnel in front of the heater closest to the concrete plug that closes the tunnel. The second array is installed within plexiglass pipes that were mounted in the roof area of FE tunnel and reach beyond the same cylindrical heater. Each night, from 22:00 to 06:00 the next morning, repeated delta signals are generated by the emitters and the respective wavefields are recorded by the receiving sensors.

 

The P and S phase first arrivals of the stacked waveforms are used then to estimate seismic velocities. The ongoing experiment is a unique longterm observation of the process within the bentonite. We show that, besides a general increase in velocities over time, the measurements resolve distinct differences in various areas within the backfill. For a ray path on the cross section, the P-wave velocity increased by approximately 100 m/s, from 550 m/s in 2015 to 642 m/s in 2023. Dividing the monitored area into the different sections, we find that seismic velocities in the floor area of the tunnel show higher values than in the roof area. Considering the entire wavefields at each sensor over time, we observe strong changes in waveform characteristics by disappearing of entire phases or changes in amplitudes. All observations reflect changes in material properties over time. The reasons for different seismic velocities in roof and floor area of the tunnel might lie in the graveled texture and swelling of particles due to increasing humidity and/or in a compaction process which is faster at the bottom of the tunnel. The process of velocity increase in the bentonite backfill is still ongoing at most of the emitter – receiver combinations.